Abstract [en]

We propose a technique for the search and calculation of complex waves in open and shielded circularly symmetric metal-dielectric waveguides with piecewise homogeneous filling by proving the existence and determination of the location of roots of the corresponding dispersion equations in the complex domain. The approach develops the method employing generalized cylindrical polynomials applied earlier to the rigorous analysis and determination of real waves.

Abstract [en]

Time-harmonic propagating modes in a perfectly electrically conducting waveguide with constant cross-section $\partial \mathrm{D}$ and axis aligned with the z-axis have the form $\mathrm{u}(\mathrm{x},\mathrm{y})\mathrm{e}^{-\mathrm{i}\omega \mathrm{t}-\mathrm{i}\gamma\mathrm{z}}$ where the cross-sectional function u satisfies the two-dimensional Helmholtz equation $(\Delta+\lambda)\mathrm{u}=0$; here $k$ denotes the wavenumber and $\lambda=\mathrm{k}^{2}-\gamma^{2}$. Propagating modes occur at values $\lambda_{1}\leq\lambda_{2}\leq\lambda_{3}\leq\cdots$ of $\lambda$ generating non-trivial solutions of the Helmholtz equation; the cutoff wavenumbers correspond to setting $\gamma$ to zero. Now suppose that axially aligned PEC structures of cross-section $\Gamma$ are inserted in the waveguide. The propagation constants are perturbed to values that may be denoted $\lambda_{1}+\Delta\lambda_{1},\lambda_{2}+\Delta\lambda_{2},\ldots$; the cutoff wavenumbers of the empty waveguide are correspondingly perturbed. In this paper we present a reliable, effective and efficient method to obtain the perturbed propagation constants. It allows us to examine the inclusion of multiple strips aligned with the z-axis, with the potential for characterizing propagation in structures with a number of small strip inserts, metamaterial-filled waveguides and so on.

Mandrik, I. E.

Arefiev, S. V.

Abstract [en]

It has always been an urgent issue for the oil and gas industry to improve oil, gas, and condensate recovery at liquid and gaseous hydrocarbon fields developed with the use of artificial formation pressure maintenance techniques that involve injection of water or water combined with other displacement agents. Therefore, due to the aforesaid issues, permanent attention should still be paid to the practical problem of optimizing the non-stationary hydrodynamic pressure applied to a reservoir by regulating the operating conditions of the production and injection wells, development process optimization in general, and water flooding in particular. The theory of Buckley and Leverett, does not take into account the loss of stability of the displacement front, which provokes a stepwise change and the triple value of water saturation. Traditionally a mathematically simplified approach was proposed-a repeatedly differentiable approximation to eliminate the “jump” in water saturation. Such a simplified solution led to negative consequences well-known from the water flooding practice, recognized by experts as “viscous instability of the displacement front” and “fractal geometry of displacement front”. The core of the issue is an attempt to predict the beginning of the stability loss of the front of oil displacement by water and to prevent its negative consequences on the water flooding process under difficult conditions of interaction of hydro-thermodynamics, capillary, molecular, inertial, and gravitational forces. In this study, catastrophe theory methods applied for the analysis of nonlinear polynomial dynamical systems are used as a novel approach. Namely, a mathematical growth model is developed and an inverse problem is formulated so that the initial coefficients of the system of differential equations for a two-phase flow can be deter mined using this model. A unified control parameter has been selected, which enables one to propose and validate a discriminant criterion for oil and water growth models for monitoring and optimizing.

Arefiev, S.V.

Abstract [en]

The theory of Buckley and Leverett does not take into account the loss of stability of the displacement front whch provokes a stepwise change and the triple value of water saturation. Traditionally a mathematically simplified approach was proposed: a differentiable approximation to eliminate the ' jump' in water saturation. Such a simplified solution led to negative consequences well-know from the water flooding practice, recognized by experts as 'viscous instability of the displacement front' and 'fractal geometry of the displacement front'.The core of the issue is to attempt to predict the beginning of the stability loss of the front of oil displacement by water and to prevent its negative effect on the water flooding process under difficult conditions of interaction of hydro-thermodynamic, capillary, molecular, inertial, and gravitational forces. In this study, catastrophe theory methods applied for the analysis of nonlinear polynomial dynamical systems are used as a novel approach. namely, a mathematical growth model is developed and an inverse problem is formulated so that the initial coefficients of the system of differential equations for a two-phase flow can be determined using this model. A unified control parameter has been selected which enables one to propose and validate a discriminant criterion for oil and water growth models for monitoring and optimization.

Abstract [en]

Metasurfaces have been extensively exploited in recent years for mantle cloaking applications. In this type of problems it is of fundamental importance to determine the connection between the metasurface geometrical parameters and the realised value of surface impedance, in order to properly design the metasurface. In this paper the surface impedance of a non homogeneous metasurface, based on a sinusoidally modulated metallic pattern is analysed.

Abstract [en]

The propagation of hybrid waves in a cylindrical anisotropic inhomogeneous nonlinear metal-dielectric waveguide is considered. The physical setting is reduced to a transmission eigenvalue problem for a system of ordinary differential equations which is new type of nonlinear eigenvalue problem where spectral parameters are the wave propagation constants. For the numerical solution, a method is proposed based on solving an auxiliary Cauchy problem (a version of the shooting method). As a result of comprehensive numerical modeling, new propagation regimes are discovered.

Abstract [en]

The paper presents the results on the use of gradient descent algorithms for constructing iterative methods for solving linear equations. A mathematically rigorous substantiation of the conver- gence of iterations to the solution of the equations is given. Numerical results demonstrating the effi- ciency of the modified iterative gradient descent method are presented.

Abstract [en]

An introduction to mathematical imaging technique for solving inverse scattering problems is given. Applications are considered to inverse waveguide problems of recontructing permittivity of layered dielectric inclusions. The solution is justified of the inverse microwave imaging by establishing one-to-one correspondence between the sought quantities and the measured noisy data.